It is known that electrical conductivity of many tumors, and in particular the malignant tumors of the mammary gland, significantly differs from the conductivity of surrounding sound tissues. This fact may be used for the detection and localization of such tumors.
Some devices are known for visualizing the spatial distribution of conductivity (impedance) in the human body, e.g., electrical impedance tomographs. (A. V. Korjenevsky, Yu. S. Kultiasov, V. A. Cherepenin, "Method of obtaining of tomographic image of a body and electrical impedance tomograph," International application PCT/RU97/00398.) In an electrical impedance tomograph, the source of alternating electric current is connected to pairs of electrodes placed along a line on the surface of a body and measurements of the potential differences are made on the other pairs of electrodes. The results of measurements obtained from various combinations of electrodes are used to reconstruct the conductivity distribution with the help of a computer. A three-dimensional distribution of conductivity, however, is not possible with this type of tomograph. Furthermore, its resolution falls significantly from the periphery to the center of contour of the electrodes. Consequently, it is not an adequate device for the diagnosis of mammary gland tumors.
An electroimpedance mammograph device, purporting to yield a three-dimensional impedance distribution has also been described. (A. Nowakovsky, J. Wortek, and J. Stelter, "A Technical University of Gdansk Electroimpedance Mammograph," Proc. IX Int. Conf. Electrical Bio-Impedance, Heidelberg, 1995, p. 434-437; J. Wortek, J. Stelter, A. Nowakovsky, "Impedance mammograph 3D phantom studies," Proc. X Int. Conf. Electrical Bio-Impedance, Barcelona, 1998, p. 521-524.) This device contains a compact set of electrodes that are placed on the inner surface of a rigid hemisphere, multiplexers that are connected to the source of alternating electric current, and a device for measuring the potential difference between various pairs of electrodes. The measured potential differences are sent to a computer to reconstruct and display the three-dimensional impedance distribution inside the hemisphere. The serviceability of this device based on actual measurements on the human body have not yet been published.
A number of limitations for this device are apparent. The arrangement of electrodes on the hemisphere surface limits the application of the device because the contact with all the electrodes can be provided only for a breast of a definite size. The device also has no means to measure the quality of each contact, nor any means to correct the reconstruction calculations if any of the electrodes have insufficient contact with the body. This decreases the validity of the data. The electric current source and the measuring device are connected to pairs of electrodes placed on the hemisphere, requiring four multiplexers. When the number of electrodes is large, these multiplexers become the most complicated and expensive parts of the device. These multiplexers are also the largest source of spurious signals due to channel-to-channel crosstalk. Only 64 electrodes were used in the prototype. This is obviously insufficient for obtaining satisfactory resolution. However, further increasing this number is problematical because of the difficulty of commutation by the chosen measuring circuit.
When a safe level of electrical current is used, the measured potential differences are small because of the small distances between electrodes. This decreases the signal-to-noise ratio and the display quality. This effect only gets worse with an increasing number of electrodes. The algorithm for image reconstruction is based on the perturbation method. It requires a calculation time of ten minutes on a workstation for a system of 64 electrodes (Nowakovsky). For 180 electrodes, the minimum necessary for practical applications, about ten hours are required for the calculations based on the estimations of the authors. This amount of time is not feasible for clinical practice.
Other attempts at obtaining three-dimensional electrical impedance images of a body have involved multi-dimensional electrode arrays that surround or partially surround the object being imaged. The reconstruction algorithms used and the methodology of making measurements were also different than those of the present invention. These efforts include U.S. Pat. Nos. 5,284,142, 5,351,697, 4,263,920, 4,486,835, and 4,539,640.
A goal of the present invention is to provide a new method and apparatus for electric mammography with greater reliability and accuracy of measurements, higher resolution, and increased computational speed. This invention is particularly applicable to the clinical diagnosis of breast tumors and other subsurface areas.